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Agronomy QUESTION #2091
Question 1
Heat stress during grain filling in wheat causes yield loss primarily through:
  • Delayed heading and extended vegetative growth phase
  • Accelerated leaf senescence, shortened grain-filling duration, and reduced starch synthase enzyme activity — resulting in smaller, shriveled grains with lower test weight even when adequate assimilates are available✔️
  • Increased tiller mortality and reduced spike number
  • Enhanced water use reducing soil moisture below wilting point
Correct Answer Explanation
Heat stress (>30°C during grain filling) directly impairs starch biosynthesis: starch synthase is heat-labile, losing activity above 30°C. This shortens effective grain-filling duration (fewer days × lower daily fill rate), producing small, shriveled grains. Agronomic responses include early planting (escape strategy), irrigation to reduce canopy temperature, and selection of heat-tolerant varieties with thermostable starch synthase.